LIGAND SUBSTITUTION AT 19-ELECTRON CENTERS AND THE INDENYL EFFECT IN ORGANOMETALLIC RADICALS - ELECTROCATALYTIC CO SUBSTITUTION IN (CYCLOPENTADIENYL)FE(CO)(3)+ AND (INDENYL)FE(CO)(3)+

Electrochemical reduction of (eta(5)-Cp)Fe(CO)(3)(+) (1(+)) and (eta(5 )-indenyl)Fe(CO)(3)(+) (2(+)) in the presence of P- and As-donor nucle ophiles (L) leads to rapid and efficient CO substitution by an electro n-transfer-catalyzed (ETC) pathway to afford (eta(5)-Cp)Fe(CO)(2)L(+) and (eta(5)-indenyl)Fe(CO)(2)L(+). The CO substitution may also be eff ected quantitatively and rapidly by using trace amounts of chemical re ducing agents such as NEt(3) and Na/Pb. A detailed variable-temperatur e electrochemical study showed that the 19-electron radical 1 dissocia tes CO with a rate constant greater than 10(3) s-(1) at -112 degrees C in butyronitrile. In contrast, 2 is relatively stable, with k-(co) be ing at least 10(6) times less than that for 1. Voltammetry with conven tional electrodes and with microelectrodes under steady-state conditio ns allowed the mechanism of CO substitution in the 19-electron radical s 1 and 2 to be established as strictly dissociative. This fact, as we ll as the determination (from microelectrode steady-state experiments) that the rate of heterogeneous charge transfer for the process 2(+) - -> 2 is fast while that for 2 --> 2(-) is slow, argues strongly that t he indenyl ligand in 2 is eta(5)-bonded and not eta(3)-bonded as previ ously proposed. The results of extended Huckel MO calculations provide a simple explanation of the reduced reactivity of 2 in comparison to 1, without the necessity of invoking ring slippage. The LUMO's of 1(+) and 2(+) contain a large amount of metal character, as was confirmed by an examination of the ESR. spectrum of 2. The LUMO's of 1(+) and 2( +) are both Fe-CO antibonding and have a similar amount of metal chara cter but differ in that the LUMO of 2(+) has a significant localizatio n on the benzene ring of the indenyl ligand, with proportionately less localization on the CO ligands. Accordingly, the rate of dissociation of CO is much greater for 1 than for 2. The origin of this effect can be traced to the presence of a low-lying: pi orbital in the indenyl anion that is predominantly localized on the benzene ring and which ha s the proper symmetry to interact with one of the two LUMO's on the Fe (CO)32+ fragment. In effect, the indenyl ligand acts like an electron sink when 2(+) is reduced, but this involves neither ring slippage to eta(3) bonding nor a diminution of electron density on the metal in co mparison to that in 1. The principal conclusion is that changing from cyclopentadienyl to indenyl ligands should greatly retard dissociative substitutions at 19e(-) centers (inverse ''indenyl effect''), while, for similar reasons, substitutions should be accelerated at 18e(-) cen ters (A or D mechanism) and 17e(-) centers (A mechanism), in accordanc e with the well-known indenyl effect. Also reported in this study is t he X-ray structure of [(eta(5)-indenyl)Fe(CO)(3)]PF6: orthorhombic, sp ace group Pnma, a = 9.7911(8) Angstrom, b= 7.5975(11) Angstrom, c = 19 .909(2) Angstrom, Z = 4, 1405 unique reflections, R1 = 0.075, wR2 = 0. 204.